The present invention relates to a reversal preventing device of an electric clock mechanism constructed in such a way that the pointers are shifted intermittently by a step motor, wherein the vibration of the second hand has been remarkably reduced.
Heretofore, in an electric clock provided with a step motor having a rotor which is rotated by a definite angle in one direction upon each driving pulse, a drive train was generally used wherein the pointers such as the second hand are intermittently moved by connecting the drive train of gear wheels to a pinion directly connected to the rotor. However, in such a mechanism, the movement of the rotor is transferred undamped to the second hand, so that when the rotor running in step with each driving pulse is stopped, the vibration caused by the inertia thereof is transferred undamped to the second hand and appears in the second hand movement as a vibration. This vibration makes it difficult to read the time.
In order to solve this defect, two methods were hitherto considered, one is a method of loading frictional pieces on the train of gear wheels directly connected to the second hand, or of making use of the action of click pieces. But in such a device the output of the step motor must be large enough to ensure driving of the resulting load which is larger than that necessary to drive just the train of gear wheels; moreover, in case a battery is used as the electric power source, there is a disadvantage in that its life is reduced.
The other method consists in reducing the inertia of the rotor. But in this method, it is needed to minimize the size of the motor as far as possible, which leads to a small output torque of the motor resulting in a decreased reliability. In addition, to elevate this torque characteristic, it is necessary to use magnetic materials for obtaining high energy, which is disadvantageous in that the cost is increased.
Consequently, this invention aims at eliminating the above-mentioned defects without degrading the starting characteristic of the step motor.